1. Field of the Invention
The present invention relates to a power steering device for a saddle-ridden vehicle. More particularly, to a power steering device for a saddle-ridden vehicle having a differential device.
2. Description of Background Art
Four wheel drive vehicles are know that include a differential locking device (hereinafter referred to as a “diff lock device”) that puts a differential function of a differential device for causing left and right wheels to be driven in unison into a non-operable state (locked state). The diff lock device is then caused to operate as a result of a rider operating an actuator (referred to as a “diff lock actuator” in the following) of the diff lock device in order to maintain a favorable state of travel on snow-covered roads and irregular surfaces etc.
It is well-known that with vehicles equipped with the above diff lock device the force required for the operation of the steering device, i.e. the steering force becomes large as a result of the diff lock device being actuated. As a result, a power steering device such as in, for example, Utility Model Application Number Sho. 58-188742 (Utility Model Laid Open Number Sho. 60-98572) is proposed where the steering force is substantially fixed as a result of changing the assist force of a hydraulic power steering device in correlation with the operation of the diff lock device. In recent years, electric power steering devices (hereinafter referred to as “EPS”) where assist force of a power steering device is obtained using an electric motor are well-known. See, for example, Japanese Patent Publication Laid-open No. 2007-98984.
Current supplied by the electric power steering device to the motor in order to generate a substantial assist force is large in order to maintain the same steering force at the time of diff locking as at times other than the time of diff locking. When diff locking where the current supplied to the motor is large is continued, the temperature of the electric power steering device becomes high. When the temperature of the electric power steering device becomes a prescribed value or more, it can be considered to stop the operation of the power steering device until the temperature falls. However, there are also cases where the temperature of the electric power steering device does not fall below a prescribed value even after the diff lock is released from the necessary travelling state. In such cases, there is a problem that the assist function for the steering force due to the electric power steering device cannot be restored until the temperature of the electric power steering device falls to a prescribed value or less. This causes the handle operation to be continually heavy.